So I will say what it is to me: It is a stage.
A stage where you play your chosen part in a play without a script, where every action or inaction of every player affects every other player - although only the actions of those close to you are immediately obvious.
Some "players" only dip their toes in the waters, preferring to skim lightly across the surface. Others dive deep down, and are absorbed in the ocean. There are those that use SL to play games, others who use it to make artworks, there are even theater performances within the fictional world.
For my own part, I live here. I do not play a role, I am that role - that of Lawra Idlemind, Mistress of the Moirai family. When I stare frostily at an offending slave it is not an act, it is my own reaction. When I smile sweetly and forgive, that is also totally real to me. When I was commended for my "good role play" recently I did a double-take - I hadn't been acting at all!
This makes it all the more enjoyable to me: The challenge of living the life of a Mistress is far more interesting than merely acting the part. So even if I am fully aware that I am a fictional character controlled by a person behind a keyboard somewhere in Norway, I am still totally convinced that I really am - myself.
I have discovered that how I react ("play my part") is very much influenced by the costume I wear. As the tall blonde in the virginally white outfit I am sweet, caring and forgiving. In full spiky goth armour, skin and hair, and riding Spike (my Gothic horse) I am cold, cruel and utterly unforgiving. My dark-skinned wide-hipped big-bosomed form is unabashedly erotic, and can make any outfit from bikini to ballgowns look sexy enough to border on the slutty.
All the world's a stage,
And all the men and women merely players:
